1. Field of the Invention
The present invention relates to a voltage regulator which generates a constant output voltage Vout upon receiving an input voltage, and more specifically to transient response characteristics and stable operation of a voltage regulator.
2. Description of the Related Art
In general, a voltage regulator generates a constant output voltage Vout to an output terminal 16 upon receiving an input voltage Vin which is input to an input terminal 15. The voltage regulator supplies electric current in response to load fluctuations to maintain the output voltage Vout constant consistently.
FIG. 2 is a circuit diagram of a conventional voltage regulator.
A reference voltage circuit 110 generates a reference voltage Vref. Bleeder resistors 111 and 112 divide the output voltage Vout of the output terminal 16 to generate a feedback voltage Vfb. The reference voltage Vref and the feedback voltage Vfb are input to an input terminal of a differential amplifier 120. An output voltage of the differential amplifier 120 is input to a gate terminal of a MOS transistor 123 which constitutes a first source ground amplifier circuit. The MOS transistor 123 has a source terminal connected to the input terminal 15 and a drain terminal connected to a constant current source 124, a resistor 121, and a capacitor 122. An output of the MOS transistor 123 is input to a gate terminal of a MOS transistor 114, which constitutes a second source ground amplifier circuit, via the resistor 121. The MOS transistor 114 has a source terminal connected to the input terminal 15 and a drain terminal connected to the bleeder resistor 111. The output terminal 16 of the voltage regulator is a contact between the MOS transistor 114 and the bleeder resistor 111. The output terminal 16 of the voltage regulator is connected to a load capacitor CL and to a load having a load resistor RL.
The operation of the conventional voltage regulator will be described below.
If the reference voltage Vref is greater than the feedback voltage Vfb, the output of the differential amplifier 120 is high, which increases the ON resistance of the MOS transistor 123. If the ON resistance of the MOS transistor 123 increases, the voltage at the gate terminal of the MOS transistor 114 decreases via the resistor 121. Since the ON resistance of the MOS transistor 114 decreases, the output voltage Vout increases. Therefore, the voltage regulator operates such that the feedback voltage Vfb equals the reference voltage Vref. If the feedback voltage Vfb is greater than the reference voltage Vref, the operation is performed in a manner opposite to the above and thus the output voltage Vout decreases.
The voltage regulator always maintains the feedback voltage Vfb and the reference voltage Vref equal to each other, thereby generating a constant output voltage Vout.
The voltage regulator requires a wide frequency band in order to improve transient response characteristics. The conventional voltage regulator employs a voltage three-stage amplifier circuit configuration to improve transient response characteristics by using a wide frequency band even in the case of relatively less consumption current. The voltage three-stage amplifier circuit configuration, however, causes a phase delay of 180 degrees or more, by which the voltage regulator is susceptible to unstable operation such as oscillation. Therefore, the conventional voltage regulator additionally has the resistor 121 and the capacitor 122. The phase delay, which occurs in the voltage three-stage amplifier circuit, is compensated by generating a zero point by the resistor 121 and a parasitic capacitance of the MOS transistor 114 to maintain stable operation (for example, refer to Patent Document 1).
[Patent Document 1] Japanese Patent Application Laid-Open No. 2005-215897